1. Field of the Invention
The present invention relates to sintered hard metal compacts for use in earth-boring bits, specifically to the composition of binder matrix materials for use in such sintered hard metal compacts.
2. Summary of the Prior Art
Sintered hard metal compacts long have been used in earth-boring bits to provide such bits with wear-resistance and increased earth-disintegrating ability. Many of these compacts comprise carbides of the group IVB, VB, VIB, or VIIB metals. The carbides are sintered into solid solution with one or more transition metals selected from group VIII. The transition metals thus form a binder matrix for the carbide particles. Depending on the composition of such a hard metal compact, various desirable mechanical properties, such as fracture-toughness, wear-resistance, and hardness are obtained.
The group VIII transition metal cobalt makes an excellent binder matrix material because it has excellent wetting properties in its liquid state. Its wetting ability permits cobalt to distribute itself better over carbide particles, thus providing an excellent binder matrix material having high toughness. However, cobalt is somewhat rare and more expensive than other metals, and can be difficult to obtain. Therefore, the bulk of recent effort in hard metal technology is to find a substitute for cobalt in the binder matrices of such hard metals.
U.S. Pat. No. 3,245,763, Apr. 12, 1966 to Ohlsson et al. discloses a hard metal alloy having superior qualities even if cobalt is wholly or partly substituted by nickel and/or iron. U.S Pat. No. 3,384,465, May 21, 1968 to Humenik, Jr., et al. discloses a sintered compact of tungsten carbide with a binder matrix of iron and nickel in place of cobalt-based material. U.S. Pat. No. 3,816,081, Jun. 11, 1974 to Hale discloses a hard metal having a binder matrix comprised mostly of iron with addition of up to 15 weight percent cobalt and 20 weight-percent nickel. U.S. Pat. No. 3,993,446, Nov. 23, 1976 to Okawa discloses a hard metal with a binder matrix comprised of nickel and cobalt, preferably in the ratio of 2:1. U.S. Pat. No. 4,947,945, Aug. 14, 1990 to Griffin discloses a cutting element for use in an earth-boring bit having a binder matrix comprising nickel and iron.
Most of the foregoing references disclose compositions directed toward replacement of cobalt in the binder matrices of sintered hard metals while retaining objectively good mechanical properties such as hardness, wear-resistance, and fracture-toughness. None of these materials have proved particularly successful for use as compacts in earth-boring bits.
This lack of success may be attributable to the fact that the cutting dynamics of earth-boring bits, and the loading and wear experienced by sintered hard metal compacts used in such earth-boring bits, are not fully understood. Therefore, the utility of a material quantified only by measured mechanical properties is dubious because the exact combination of desirable mechanical properties for sintered hard metal compacts for use in earth-boring bits also is not fully understood.
It has been found that a hard, sintered compact having a binder matrix comprised of nickel and cobalt yields an improved compact that is well-suited to the demanding environment present in earth-boring bit applications. The present invention employs a ratio of cobalt to nickel in the binder matrix that is substantially higher than that disclosed in the prior art, which is directed toward decreasing the quantity of cobalt in such compacts, as discussed above.